Implantable prostheses are commonly used to replace or augment body tissue. Such prosthesis include a shell formed of an elastomeric material, e.g., silicone. The shell is filled with filling material such as saline or some other liquid or a gel. The filling of the shell commonly takes place after the shell is inserted through an incision. The shell includes a valve that can accept a filling tube that passes through the incision and is used to fill the shell with a suitable liquid or gel. Once the shell is filled to the desired degree, the filling tube is removed and the incision is closed.
In the case of the female breast, it sometimes necessary to remove some or all of the mammary gland and surrounding tissue in order to treat breast cancer. This surgery leaves a void that can be filled with an implantable prosthesis. The implant serves to support surrounding tissue and to maintain the appearance of the body. The restoration of the normal appearance of the body has an extremely beneficial psychological effect on post-operative patients, eliminating much of the shock and depression that often follows extensive surgical procedures.
Implantable prostheses are also used more generally for restoring the normal appearance of soft tissue in various areas of the body.
Tissue expanders generally resemble implantable prostheses except that they include a means for adding additional liquid or gel after the device had been inserted under the skin and the incision has been closed. After implantation the shell is gradually inflated using a liquid or gel, usually over a period of weeks, in order to expand the overlying skin either so that a prosthesis can later be inserted or so that skin can be generated for grafting. The liquid or gel is usually introduced by means of a needle that pierces the skin and a self-seal valve that is integral to the shell or that is remote from the shell and connected to the shell by tubing.
Silicone shells for implantable prosthesis and tissue expanders are generally formed by dipping a suitably shaped mandrel into a silicone dispersion. The mandrel is withdrawn from the dispersion and the excess silicone dispersion is allowed to drain from the mandrel. After the excess dispersion has drained from the mandrel at least a portion of the solvent is allowed to evaporate to stabilize the silicone coating. The process is then repeated one or more times until a shell of the desired thickness is formed. Because the flow of the silicone dispersion as it drains from the mandrel depends on the shape and orientation of the mandrel, the resulting shell can vary substantially in thickness. In addition, because dip casting requires a relatively large vat of silicone dispersion and because solvent evaporates from the silicone dispersion in the vat during the casting process, considerable silicone dispersion waste is created during dip casting.